This text presents the findings of a major investigation of second language proficiency in various groups of school-aged learners. The paperback edition presents the findings of a major investigation of second language proficiency in various groups of school-aged learners, and invites a frank appraisal of the research from an outside panel of experts. The theoretical and practical implications of the study are the subject of a lively debate focused on three main research issues: the nature of second language proficiency, the impact of classroom treatment on second language learning, and the role of social and individual factors in bilingual development. This original collection of articles will be of interest to all those concerned with policy and practice in second language education.